In Boudin’s bread museum, I learn about sourdough’s storied past, how it arrived with the miners during the Gold Rush. How a French immigrant named Isidore Boudin launched the bakery in 1849 with his own sourdough starter, and how for more than 150 years Boudin bakers have kept this living, fermenting, microorganism-rich mix of flour and water alive. It’s used in every batch of their sourdough, several thousand loaves a day, including the one tucked under my arm.

I look through a microscope and see the Tic Tac–shaped bacterium responsible for sourdough’s tang. Lactobacillus sanfranciscensis was identified in 1971, at a USDA lab across the bay in Albany, and named in honor of the city’s famous bread. It “thrives only in San Francisco’s climate,” reads a nearby placard. “My, my!” murmurs a woman next to me. We’re seeing the secrets of a San Francisco icon.

Or are we?

Scientists have since discovered that sourdoughs—that is, any bread made with a “sour” dough as opposed to instant yeast—are full of L. sanfranciscensis. That means German sauerteig, French levain, Ethiopian injera, and dozens of other bread varieties are, technically, sourdough.

“I don’t want to burst any bubbles, but that’s just a general rule found across the microbial world,” says Maria Marco, a microbiologist and professor of food science and technology at the University of California, Davis. So is there no such thing as San Francisco sourdough?

The question rises in my mind at Berkeley’s Acme Bread Company. Thirty years ago, at a time when cottony white instant-yeast loaves dominated the bread scene, baker Steve Sullivan led the West Coast artisanal bread revolution, powered by sourdough. Using a French-style levain starter made with wild yeast from grapes, he resuscitated the old, slow, flavorful ways of sourdough for an eager new audience. Today Acme makes thousands of loaves a week that fill breadboxes all over the Bay Area.